Method of fabricating spheroidal graphite cast iron parts of high precision, geometrically and dimensionally, and having improved mechanical characteristics

ABSTRACT

Disclosed is a method of fabricating spheroidal graphite cast iron parts including a) preparing a mixture in the liquid state having the following composition by weight: 3% to 4% C; 1.7% to 3% Si; 0.1% to 0.7% Mn; 0 to 4% Ni; 0 to 1.5% Cu; 0 to 0.5% Mo; with a residual Mg content adapted to the thickness of the parts and lying in the range 0.025% to 0.080%; the balance being iron and impurities; b) casting this liquid mixture at a temperature in the range 1350° C. to 1550° C. into a mold to obtain a blank; c) extracting the blank from the mold at a temperature between the solidus and AR3, where the solidus and AR3 represent the limit temperatures for the austenitic range of the composition, or at a temperature less than AR3; d) shaping the blank at a temperature in the range 1050° C. to AR3; e) cooling to a temperature lying in the range 260° C. to 420° C. situated in the bainitic range, and maintaining this temperature for 60 min to 180 min, and cooling to ambient temperature.

The present invention relates to a method of fabricating castings of spheroidal graphite cast iron, and to cast iron as obtained by implementing the method.

The term “high precision, geometrically and dimensionally, and of improved mechanical characteristics” is used herein to mean spheroidal graphite cast iron parts of surface state, of geometrical and dimensional precision, and of mechanical characteristics that are better than those normally encountered with conventional casting methods.

French patent No. 2 839 727 discloses a specific method of fabricating spheroidal graphite cast iron parts which combines the techniques of casting, forging, and heat treatment by associating them in succession. The method described in that patent enables spheroidal graphite cast iron parts to be obtained having very good mechanical characteristics (traction strength Rm>1000 megapascals (MPa); ratio of the elastic limit over traction strength Rp 0.2/Rm>0.68; and breaking elongation A % lying in the range 4% to 14%) with dimensional and geometrical precision close to that of conventional forging, i.e. better than that obtained by conventional casting techniques.

Nevertheless, in that patented method, all of the fabrication steps are performed in the heat of casting; that requires intermediate operations of maintaining temperature between certain fabrication stages, since not all of the steps have the same duration. This applies in particular to the operations of casting and forging, and also to forging and heat treatment by bainitic staged quenching. Implementing the method thus requires a specific and expensive production line, i.e. the method is well adapted to mass production, e.g. of the automobile type, but is poorly adapted to medium and short runs.

Furthermore, numerous mechanical parts do not require mechanical characteristics of the high grade obtained by patent FR 2 839 727 with bainitic or austeno-ferritic type structures, which are also difficult to machine.

In applying the method to certain types of parts that require high plastic deformation ratios for a stamping operation, it has also been found that the parts suffer damage during trimming operations because graphite nodules are sometimes subjected to large amounts of deformation and are in alignment in certain zones of the parts close to the seam of flash and in the seam of flash itself, tending to allow the metal to tear in those locations when a cutting tool passes therethrough.

In addition, for parts of certain shapes, the weak point in fatigue stressing is situated in the flash that is located in the join plane that is generally situated in the middle portion of the part.

When applying the method of patent FR 2 839 727, it can be difficult to control the flash that is inherent to stamping in open dies, and thereby control the deformation ratio of the cast iron in those locations, and thus the deformation ratio of the graphite nodules.

Furthermore, manufactures are seeking more and more to reduce the purchase price of rough parts, and they are requiring rough parts that are more and more precise, i.e. that need as little machining as possible.

Under such conditions, in order to be economically more competitive than the conventional method of forging steel parts, in particular for fabricating mass-produced parts of the automotive type, and also for fabricating parts in shorter runs, the patented method needs to be improved.

The object of the present invention is to satisfy this object and to remedy those drawbacks by proposing a method of fabricating spheroidal graphite cast iron parts having the following characteristics:

-   -   yield that is improved compared with the conventional forging         method and with the earlier patent FR 2 939 727;     -   fatigue limit improved relative to that obtained by the method         of patent FR 2 839 727;     -   mechanical characteristics better than those obtained by a         conventional casting method; and     -   dimensional and geometrical precision that are better than those         obtained by conventional casting and forging methods, and better         than those obtained with the method constituting the subject         matter of patent FR 2 839 727.

The spheroidal graphite cast iron parts obtained in this way possess dimensional and geometrical precision that is sufficient, in some cases, to eliminate pre-machining operations on reference surfaces.

To this end, the invention provides a method of fabricating spheroidal graphite cast iron parts of high dimensional and geometrical precision, and having improved mechanical characteristics, the method comprising the following steps:

-   -   a) preparing a mixture in the liquid state having the following         composition by weight: 3% to 4% C; 1.7% to 3% Si; 0.1% to 0.7%         Mn; 0 to 4% Ni; 0 to 1.5% Cu; 0 to 0.5% Mo; with a residual Mg         content adapted to the thickness of the parts and lying in the         range 0.025% to 0.080%; the balance being iron and impurities         resulting from preparation; the impurities being in particular S         at a content of less than 0.015% and P at a content of less than         0.10%;     -   b) casting this mixture in the liquid state at a temperature         lying in the range between 1350° C. and 1550° C. into a mold to         obtain a blank of the part that is to be obtained, which blank         is of a shape close to the shape of the part;     -   c) extracting said blank from the mold at a temperature Ts lying         between the solidus and AR3, where the solidus and AR3 represent         the limit temperatures for the austenitic range of said         composition;     -   d) shaping the blank at a temperature Tf lying in the range         between 1050° C. and AR3, by hot plastic deformation, directly         in the heat of casting or after being maintained at temperature         Tm=Tf+20° C. to 50° C. for a duration lying in the range between         10 minutes (min) and 60 min, in order to obtain the part in its         final shape and dimensions;     -   e) quenching said part directly in the heat of forming at a         temperature Tb lying in the range between 260° C. and 420° C.         and situated in the bainitic range, and maintaining the part at         said temperature Tb for a duration tb lying in the range between         60 min and 180 min; and     -   f) cooling said part to ambient temperature;     -   the method being characterized in that the blank obtained by         molding possesses a volume substantially identical to that of         the part, and the shaping operation by hot plastic deformation         is a calibration operation in closed containers or dies enabling         the calibrated cast part to be obtained without any lateral         flash.

This method of implementing the method of the invention makes it possible to obtain spheroidal graphite cast iron with structure that is essentially bainitic, thereby conferring very good mechanical characteristics to the parts, but with a fatigue limit that is greater than that obtained using the method of patent FR 2 839 727.

The term “essentially bainitic” is used herein to mean a structure constituted by at least 50% bainite.

In another implementation of the method of the invention, after the operation of calibration by hot plastic deformation to obtain the calibrated molded part without any lateral flash, said calibrated molded part is cooled to ambient temperature in free manner, under control or by quenching, in order to confer the desired mechanical characteristics to the part; this replaces bainitic staged quenching. This implementation of the invention makes it possible to obtain spheroidal graphite cast iron with a structure that is constituted by varying proportions of ferrite and/or perlite; the structure is then essentially ferritic or essentially perlitic, or ferrito-perlitic.

The terms “essentially ferritic” and “essentially perlitic” are used herein, and throughout the text below, to designate a structure constituted respectively by at least 50% ferrite or at least 50% perlite.

The mechanical characteristics as obtained for the parts, and in particular the fatigue limit, although they are less good than those that result from applying the method of patent FR 2 839 727, are better than those to be found on spheroidal graphite cast iron parts that have merely been cast, with this being due to the compacting and work-hardening effect obtained by the calibration operation.

In these first two implementations of the method of the invention, between the blank leaving the mold c) and the operation of hot plastic deformation d), or between the blank leaving the mold c) and the additional operation of keeping the blank at the temperature Tm, an intermediate operation is added of separating casting heads (feeders, chutes, . . . ) from the blank by cutting or by some other method, in order to obtain the blank alone with a volume that is substantially identical to the volume of the calibrated casting that is to be obtained.

This intermediate operation is intended to remove all of the casting heads from the cast blank, and also, if necessary, to improve its shape or its volume, in preparation for the following calibration operation. This operation may also include an operation of cleaning said blank of the part, e.g. by brushing or by some other method, in order to remove therefrom any oxides or residues coming from the casting operation, prior to performing the calibration operation.

In another implementation of the method of the invention, after the operation of casting the blank and before the operation of calibrating it by hot plastic deformation in containers or in closed dies, said blank is extracted from the mold after cooling to below AR3, and it is finished at ambient temperature, and said blank is reheated and maintained at a temperature in the range AC3 to 1050° C. for a duration lying in the range 10 min to 90 min to ensure good uniformity of temperature and chemical composition inside said blank.

Finishing the cast blank, which includes removing heads, grinding, and possible operations of deburring and of shot-blasting, is made easier in this implementation of the method of the invention, since it is performed at ambient temperature; the method is thus simplified in terms of implementation, since the molding and calibration operations are dissociated.

In this implementation of the method of the invention, it is also possible for the finishing operation to include an operation of grinding and/or cutting that is intended to obtain a volume for said blank that is substantially identical to the volume of the part and/or that is intended to readjust the shape of said blank.

In the various implementations of the method of the invention, the plastic deformation ratio during the calibration operation is deliberately restricted in order to avoid excessive flattening of the graphite nodules which become oriented in the flow direction of the metal.

Eliminating lateral flash by using a blank of volume substantially identical to that of the part to be obtained and by using closed dies has the effect of eliminating the zones of weakness in fatigue stress due to the nodules at such locations being subjected to large amounts of deformation.

Furthermore, the operation of cutting off lateral flash is no longer necessary, and the risks of metal tearing in these zones are eliminated.

In the various implementations of the method of the invention, the shape given to the cast blank is determined from the final shape required for the part so as to allow for a deformation ratio in all of the portions of the blank during the calibration operation that lies in the range 1% to a maximum of 20%, and to direct metal flow during this operation in a direction that is favorable to mechanical characteristics, and in particular to the fatigue stressing of the part in operation.

According to other characteristics of the invention in various implementations of the method:

-   -   the operation of calibration in closed containers or dies is         implemented by pressing or striking the blank between at least         two tools or by using moving elements and/or punches sliding         inside the dies or the tools, in particular in order to obtain         multiaxial plastic deformation of the blank;     -   the volume of the cast blank is equal to the volume of the         calibrated casting, with maximum tolerance lying in the range 0         to +6%;     -   the volume of excess material on the blank corresponding to         allowable tolerance on the volume of the blank is directed,         during the calibration operation, into an excess metal housing         placed either in one or more blind holes or through holes in the         part that are to be machined, or in one or more cavities         provided for this purpose in the part in zones that do not         affect the geometrical precision of the calibrated casting; and     -   the mold used for casting the blank of the part is preferably a         permanent mold constituted by at least two metal half-portions         coated in a release agent, but it could also be a non-permanent         mold of sand or other material.

In implementations of the method of the invention that include cooling the calibrated casting in free manner, under control, or by quenching, the cooling is performed in free air, in blown air, or in a confined medium or atmosphere, depending on circumstances.

The term “cooling in controlled manner” is used herein to mean cooling performed at a speed that is set and controlled so as to enable a previously-defined and desired structure to be obtained for the part. When the defined and desired structure is not obtained directly following cooling performed in free manner, under control, or by quenching, an annealing heat treatment operation is also included after the operation of cooling the calibrated casting, for the purpose of adjusting the structure and/or the mechanical characteristics of said calibrated casting.

The invention also provides spheroidal graphite cast iron prepared and shaped using the method of the invention and of structure that is either essentially bainitic or essentially perlitic or essentially ferritic, or is constituted by varying portions of ferrite and perlite.

The invention also provides a cast iron part constituted by spheroidal graphite cast iron prepared and shaped using any of the implementations of the method of the invention.

The present invention is particularly adapted, without being limited thereto, to fabricating mechanical parts for automobile engines, such as connecting rods or other moving parts of an engine.

Being lighter in weight than forged steel, and having mechanical characteristics that are similar to or even better than those of forged steel, spheroidal graphite cast iron parts that have been molded and calibrated by hot plastic deformation present a surface state, and dimensional and geometrical precision, that are better than those obtained with conventional methods of mass-production forging or casting.

The invention is particularly well adapted, although limited, to parts that need to present a high fatigue limit, with a saving in weight compared with steel.

Another major advantage of the invention is to make it possible to make spheroidal graphite cast iron parts of shapes that are impossible to obtain for steel parts made by conventional stamping, such as, for example, a part having hollow shapes situated perpendicularly to the join plane, and thus undercut relative to the striking or pressing direction. These shapes are obtained easily by casting in a mold, and they are already present in the cast blank of the part, which then requires to be subjected to plastic deformation ratios that are small only, in order for the part to be put into its final shape; these small plastic deformation ratios are compatible with a hot calibration operation in containers or closed dies with inserts or tools placed in the dies, as is performed in the various implementations of the invention.

By way of non-limiting example, mention can be made of a connecting rod for an engine, in which rod the web, i.e. the central portion of the rod interconnecting its big and little ends, has an H-shaped profile in which the hollows extend in the direction of the join plane, i.e. in the direction that is suitable for stamping. For better service life, the profile ought to, be placed perpendicularly to the present profile relative to the part, but that cannot be done with conventional stamping techniques on a steel part. The method of the invention enables an engine connecting rod to be made with a web of profile that is perpendicular to present-day profiles, i.e. with the hollow shapes in the section of the web placed perpendicularly relative to the join plane, thus giving the part better service life under the mechanical stresses of operation.

Other characteristics and advantages of the present invention appear on reading the following description of an implementation that is given by way of non-limiting example.

Tests have been performed on an automobile connecting rod having a weight of 770 grams (g), of the kind conventionally made of forged steel, but fabricated in accordance with the invention. In that part, the longest dimension was 180 millimeters (mm) and its greatest thickness was 25 mm.

In accordance with the invention, starting from the desired final shape for the calibrated cast connecting rod, we determined the shape to be given to the blank of the cast connecting rod suitable for ensuring a deformation ratio of said blank during the calibration operation lying in the range from 1% to 20% maximum in all portions thereof, for a blank of volume substantially identical to that of the desired calibrated cast connecting rod in order to obtain a calibrated cast connecting rod without any lateral flash. For performing deformation calculations, we selected a volume for the blank equal to the volume of the calibrated casting plus 2%.

By investigating plastic deformation using digital software for stimulating forging, we determined the geometrical shape and the dimensions to be given to the blank of the connecting rod. Starting from this shape for the blank, we dimensioned the shapes for calibration tooling in the form of closed dies, and also the shapes for casting tooling made up of two metal half-shells, with account being taken of cooling shrinkage of the connecting rod blank and of the calibrated cast connecting rod, and with account also being taken of expansions of the casting and calibration tooling.

The calibration tooling comprised two closed dies and an element sliding in one of the dies to make the bore in the big end; housings for excess metal were provided at various locations on the perimeter of the bore, wherever a subsequent machining operation was to take place, and also in a cavity at the little end, where a subsequent machining operation also takes place.

In accordance with the invention, the cast iron having a final composition of: 3.7% C; 2.62% Si; 0.19% Mn; 0.3% Ni; 0.4% Cu; 0.15% Mo; with a residual magnesium content lying in the range from 0.028% to 0.042%; was prepared in an electrical induction furnace, was treated with ferro-silico-magnesium, and was subsequently cast at a temperature in the range from 1380° C. to 1450° C. into a metal mold of temperature regulated at 270° C. and coated in protective release agent.

The metal mold had four cavities for connecting rod blanks, so that four connecting rod blanks of spheroidal graphite cast iron were cast during each mold cycle. In accordance with the invention, the cast connecting rod blanks were extracted from the mold at a temperature that remained in the range from 980° C. to 950° C. throughout testing.

Immediately after being extracted from the mold, each casting cluster made up of four connecting rod blanks was placed in a cutter tool mounted on a press to separate the headers and the casting feeders.

Each connecting rod blank was then placed immediately in a furnace in a nitrogen atmosphere regulated to a temperature of 950° C. so as to ensure temperature uniformity throughout the section of the connecting rod blank prior to calibration.

Each blank was maintained in the furnace for a period of time lying in the range 15 min to 35 min.

Thereafter, each blank was calibrated in a press by plastic deformation at a temperature of 930° C. in the previously-prepared calibration tooling.

After the calibration operation, three types of cooling were performed on three different batches of calibrated cast connecting rods:

-   -   on a first batch of connecting rods, each calibrated cast         connecting rod of the batch was allowed to cool in free air;     -   in a second batch of connecting rods, each calibrated cast         connecting rod of the batch, on leaving the calibration         operation, was immediately placed in a through tunnel oven in         which cooling speed was controlled to 20° C. per minute from         900° C. to 500° C., after which cooling was allowed to take         place in free air; and.     -   in a third batch of connecting rods, each calibrated cast         connecting rod of the batch, on leaving the calibration         operation, was immediately placed in a fluidized bed bath of         zircon sand regulated to a temperature of 360° C., and of volume         that was sufficiently great to guarantee temperature variation         in the fluidized bed of less than 5° C. each time a part was         immersed therein. Each connecting rod was thus subjected to         bainitic staged quenching at the temperature of 360° C. and was         maintained at that temperature in the fluidized bed for a         duration of 110 min, after which each connecting rod was removed         from the fluidized bed bath and allowed to cool in air to         ambient temperature in accordance with the invention.

Various measurements were performed on the connecting rods as made in this way in accordance with the invention in order to determine their yield, their geometrical and dimensional precision, and their mechanical characteristics. Thus, in order to characterize the dimensional and geometrical precision, dimensional measurements were performed concerning dimensions, roughness, and weight dispersion. For mechanical characteristics, the following measurements were performed: traction strength (Rm: breaking limit of the part when subjected to traction); elastic limit (Rp0.2), breaking elongation (A %), and traction compression fatigue limit at 3 million cycles (LF).

In terms of yield, i.e. the ratio between the total quantity of metal used and the quantity of metal represented by the parts produced (1000 kilograms (kg)), the measurements obtained are summarized in Table 1 below, in comparison with the values obtained with other methods for making the same connecting rod. TABLE 1 Method of the Method according Conventional invention to FR 2 837 727 forging method Material SG iron SG iron Steel used Quantity of 1520 1800 1650 metal used in kg Quantity of 1000 1000 1000 metal in the parts produced in kg Yield 1.52 1.8 1.65

This shows that the method of the invention makes it possible to reduce the quantity of metal used for producing the parts. This improves yield compared with earlier patent FR 2 839 727, and compared with the conventional method of forging steel parts.

Similarly, in terms of geometrical and dimensional precision, the measurements performed on connecting rods made in accordance with the invention are compared in Table 2 below with the values encountered by using other methods for fabricating the same parts. TABLE 2 Method used According to Of the patent Conventional Metal mold invention FR 2 839 727 forging casting Material used SG iron SG iron Steel SG iron Mean weight 770 770 835 770 per part in g Precision ±1.5%   ±2% ±2% ±3% concerning part weight Dimensional ±0.15 ±0.3% ±0.4 ±0.3 precision on the longest dimension of the part (180 mm) in mm Dimensional ±0.1 ±0.2 ±0.25 ±0.3 precision on the thickness of the part (25 mm) in mm Roughness Ra 5 to 6 6 to 7 7 to 8 7 to 9 in micrometers (μm) Draft angle in 0.5° to 2° 3° 3° 2° degrees

This table shows that geometrical and dimensional precision are indeed obtained that are better than that obtained with conventional casting and forging methods and better than that obtained with the method constituting the subject matter of French patent FR 2 837 727.

Furthermore, the measured values of mechanical characteristics obtained on calibrated cast connecting rods of essentially bainitic structure as obtained by staged quenching for 110 minutes at 360° C. are given in Table 3 below, in comparison with the values obtained on the same connecting rods by using other methods. TABLE 3 Method used According to Of the patent Conventional Metal mold invention FR2 839 727 forging casting Material SG iron SG iron Steel SG iron used Mean Rm in 1070 1020 915 980 MPa Mean Rp0.2 735 700 570 665 in MPa Mean A % 6.8 5.6 10.4 6.8 Mean LF in 380 345 315 345 MPa

These results show that the mechanical characteristics are improved compared with conventional casting and forging methods and compared with the method of patent FR 2 839 727. In particular, the fatigue limit, in this case determined using 3 million cycles in traction-compression, is considerably improved compared with earlier patent FR 2 839 727.

The measurements performed on batches of connecting rods having structure that was essentially perlitic or essentially ferritic are summarized in Table 4 below in comparison with the values obtained on connecting rods made using the same spheroidal graphite cast iron, cooled under the same conditions, but merely cast in a metal mold, i.e. without any calibration by hot plastic deformation. TABLE 4 Kind of cast iron Perlitic SG cast iron Ferritic SG cast iron Parts of same structure Parts of same structure Method used Of the Metal mold Of the Metal mold invention casting invention casting Mean 280 260 200 185 hardness HB Mean Rm 820 775 505 475 in MPa Mean 515 470 350 310 Rp0.2 in MPa Mean A % 4 3.8 10 9.2 Mean LF 320 295 310 290 in MPa

The parts compared herein, as obtained by the method of the invention and those made merely by casting in a metal mold possess the same metallographic structure, i.e. either mostly perlitic or mostly ferritic, since they were subjected to cooling at the same controlled rate.

The table shows that the method of the invention does indeed obtain mechanical characteristics that are better than those obtained by a conventional casting method, when using the same metallographic structure, i.e. for the same quantities of perlite and/or ferrite in the cast iron. 

1. A method of fabricating spheroidal graphite cast iron parts of improved mechanical characteristics and high precision, geometrically and dimensionally, the method comprising the following steps: a) preparing a mixture in the liquid state having the following composition by weight: 3% to 4% C; 1.7% to 3% Si; 0.1% to 0.7% Mn; 0 to 4% Ni; 0 to 1.5% Cu; 0 to 0.5% Mo; with a residual Mg content adapted to the thickness of the parts and lying in the range 0.025% to 0.080%; the balance being iron and impurities resulting from preparation; the impurities being in particular S at a content of less than 0.015% and P at a content of less than 0.10%; b) casting this mixture in the liquid state at a temperature lying in the range between 1350° C. and 1550° C. into a mold to obtain a blank of the part that is to be obtained, which blank is of a shape close to the shape of the part; c) extracting said blank from the mold at a temperature Ts lying between the solidus and AR3, where the solidus and AR3 represent the limit temperatures for the austenitic range of said composition; d) shaping the blank at a temperature Tf lying in the range between 1050° C. and AR3, by hot plastic deformation, directly in the heat of casting or after being maintained at temperature Tm=Tf+20° C. to 50° C. for a duration lying in the range between 10 min and 60 min, in order to obtain the part in its final shape and dimensions; e) quenching said part directly in the heat of forming at a temperature Tb lying in the range between 260° C. and 420° C. and situated in the bainitic range, and maintaining the part at said temperature Tb for a duration tb lying in the range between 60 min and 180 min; and f) cooling said part to ambient temperature; the method being characterized in that the blank obtained by molding possesses a volume substantially identical to that of the part, and the shaping operation by hot plastic deformation is a calibration operation in closed containers or dies enabling the calibrated cast part to be obtained without any lateral flash.
 2. A method of fabricating spheroidal graphite cast iron parts having improved mechanical characteristics and high precision, geometrically and dimensionally, which method is characterized in that it comprises the following steps: a) preparing a mixture in the liquid state having the following composition by weight: 3% to 4% C; 1.7% to 3% Si; 0.1% to 0.7% Mn; 0 to 4% Ni; 0 to 1.5% Cu; 0 to 0.5% Mo; with a residual Mg content adapted to the thickness of the parts and lying in the range 0.025% to 0.080%; the balance being iron and impurities resulting from preparation; the impurities being in particular S at a content of less than 0.015% and P at a content of less than 0.10%; b) casting this mixture in the liquid state at a temperature lying in the range between 1350° C. and 1550° C. into a mold to obtain a blank of the part that is to be obtained, which blank is of a shape close to the shape of the part and has a volume substantially identical to the volume of the part; c) extracting said blank from the mold at a temperature Ts lying between the solidus and AR3, where the solidus and AR3 represent the limit temperatures for the austenitic range of said composition; d) calibrating said blank of the part in containers or dies at a temperature Tf lying in the range between 1050° C. and AR3, by hot plastic deformation, directly in the heat of casting or after being maintained at temperature Tm=Tf+20° C. to 50° C. for a duration lying in the range 10 min to 60 min, in order to obtain the part in its final shape and dimensions; and e) cooling said calibrated casting to ambient temperature in free manner, under control, or by quenching, in order to confer the desired mechanical characteristics on the part.
 3. A method of fabricating spheroidal graphite cast iron parts having improved mechanical characteristics and high precision, geometrically and dimensionally, the method being characterized in that it comprises the following steps: a) preparing a mixture in the liquid state having the following composition by weight: 3% to 4% C; 1.7% to 3% Si; 0.1% to 0.7% Mn; 0 to 4% Ni; 0 to 1.5% Cu; 0 to 0.5% Mo; with a residual Mg content adapted to the thickness of the parts and lying in the range 0.025% to 0.080%; the balance being iron and impurities resulting from preparation; the impurities being in particular S at a content of less than 0.015% and P at a content of less than 0.10%; b) casting this mixture in the liquid state at a temperature lying in the range between 1350° C. and 1550° C. into a mold to obtain a blank of the part that is to be obtained, which blank is of a shape close to the shape of the part and has a volume substantially identical to the volume of the part; c) extracting said blank of the part from the mold after cooling to below AR3 and finishing at ambient temperature; d) heating and maintaining said blank of the part at a temperature lying in the range between AC3 and 1050° C. for a duration lying in the range 10 min to 90 min to ensure highly uniform temperature and chemical composition inside said blank; e) calibrating said blank of the part in closed containers or matrices by hot plastic deformation at a temperature lying in the range between AC3 and 1050° C. in order to obtain a calibrated casting without lateral flash; and f) cooling said calibrated casting to ambient temperature in free manner, or in controlled manner, or by quenching, depending on the desired mechanical characteristics.
 4. A method according to any one of claims 1 to 3, characterized in that the shape given to the cast blank of the part is determined from the final shape for the part so as to enable a deformation ratio to be obtained in all of the portions of the blank during the calibration operation that lies in the range 1% to 20% maximum, and to direct the flow of metal during this operation in a direction that is beneficial to the mechanical characteristics, and in particular to the fatigue stresses on the part in service.
 5. A method according to any one of claims 1 to 3, characterized in that the operation of calibration in closed containers or dies is implemented by pressing or striking the blank between at least two tools or by using moving elements and/or punches sliding inside the dies or the tools, in particular in order to obtain multiaxial plastic deformation of the blank.
 6. A method according to claim 1 or claim 2, characterized in that, between the blank leaving the mold c) and the operation of hot plastic deformation d), or between the blank leaving the mold c) and the additional operation of maintaining the blank at the temperature Tm, an intermediate operation is added of separating casting heads (feeders, chutes, . . . ) from the blank by cutting or some other method, in order to obtain the blank alone at a volume that is substantially identical to the volume of the calibrated casting that is to be obtained.
 7. A method according to any one of claims 1 to 3, characterized in that the volume of the cast blank is equal to the volume of the calibrated casting, with maximum tolerance lying in the range 0 to +6%.
 8. A method according to any one of claims 1 to 3, characterized in that the volume of excess material on the blank corresponding to allowable tolerance on the volume of the blank is directed, during the calibration operation, into an excess metal housing placed either in one or more blind holes or through holes in the part that are to be machined, or in one or more cavities provided for this purpose in the part in zones that do not affect the geometrical precision of the calibrated casting.
 9. A method according to any one of claims 1 to 3, characterized in that the mold used for casting the blank is preferably a permanent mold constituted by at least two metal half-portions coated in a release agent, but could also be a non-permanent mold of sand or other material.
 10. A method according to claim 3, characterized in that the operation of finishing the blank after it has left the mold includes, where necessary, an operation of milling or cutting in order to obtain a volume for said blank that is substantially identical to the volume of the part and/or that is designed to perfect the shape of said blank.
 11. A method according to claim 2 or claim 3, characterized in that the cooling of the calibrated casting in free manner, controlled manner, or by quenching, is performed in free air, in blown air, or in a confined medium or atmosphere.
 12. A method according to claim 2 or claim 3, characterized in that it further comprises annealing heat treatment after the operation of cooling the calibrated casting, for the purpose of adjusting the structure and/or the mechanical characteristics of said calibrated casting.
 13. A spheroidal graphite cast iron having the following composition by weight: 3% to 4% C; 1.7% to 3% Si; 0.1% to 0.7% Mn; 0 to 4% Ni; 0 to 1.5% Cu; 0 to 0.5% Mo; with a residual Mg content adapted to the thickness of the parts and lying in the range 0.025% to 0.080%; the balance being iron and impurities resulting from preparation; the impurities being in particular S at a content of less than 0.015% and P at a content of less than 0.10%, prepared and shaped in accordance with claim 1, and characterized in that it has a structure that is essentially bainitic.
 14. A spheroidal graphite cast iron having the following composition by weight: 3% to 4% C; 1.7% to 3% Si; 0.1% to 0.7% Mn; 0 to 4% Ni; 0 to 1.5% Cu; 0 to 0.5% Mo; with a residual Mg content adapted to the thickness of the parts and lying in the range 0.025% to 0.080%; the balance being iron and impurities resulting from preparation; the impurities being in particular S at a content of less than 0.015% and P at a content of less than 0.10%, prepared and shaped in accordance with any one of claims 2, 3, 11, or 12, and characterized in that it has a structure that is either essentially ferritic, or essentially perlitic, or ferrito-perlitic.
 15. A cast iron part, characterized in that it is constituted by a spheroidal graphite cast iron according to claim 13 or claim
 14. 